It is known to use rotation lasers on building sites, for example of buildings or in road building and/or earthworks. Particularly used are rotation lasers in which a laser beam (in the visible or infrared wavelength range) emitted by a laser unit generates a reference surface by deflection using a rotating deflection prism, this then providing a precise plane reference (in particular a height reference in the case of a horizontal plane).
Many of the currently existing rotation lasers in this context have a beam self-horizontalizing functionality (also known as self-leveling). Various technical solutions are known for fulfilling such a beam self-horizontalizing functionality, which, although they may be of purely mechanical nature, are nowadays nevertheless mostly based on sensors of an optical type. For example, the central component of the rotation laser (i.e. the laser core module), in particular comprising the laser unit and the rotatable deflection prism, may be suspended in a pendular fashion so that horizontalization accuracy can be achieved by employing gravity. The laser core module may, however, in this case advantageously be suspended precisely inclinably in a motorized fashion about two axes (at least slightly in a range of, for example, ±5° on an external housing of the device, and equipped with an inclination sensor or a horizontalization sensor, the display or signal of which can be read out and used as a starting value for active modification of the inclination position of the laser core module.
Depending on the level of development, known rotation sensors nowadays in this case also have a function (with a corresponding mechanism, sensors and control) for controlled desired inclination of the laser plane relative to the horizontal in one or two directions. To this end, the central component of the rotation laser, in particular comprising the laser unit and the rotatable deflection prism, may be inclined in a controlled motorized fashion about one axis or two axes and brought into desired inclination positions, so that the rotation axis and consequently also the spanned plane are thereby also inclined in the desired way. Corresponding mechanisms, sensors and controls for this are well known in the prior art and are described, for example, in the patent literature publications U.S. Pat. No. 5,485,266 A, US 2004/0125356 A1, EP 1 790 940 A2, EP 1 901 034 A2, EP 2 327 958 A1 and EP 2 522 954 A1.
If the rotating laser beam emitted by the rotation laser is in this case emitted in the visible spectrum and it strikes a surface, for example, a wall, a floor or a ceiling of a building, a reference line as a basis of further measures is visible there.
For precise transmission of the reference plane or reference height defined by the rotating laser beam, for example onto a wall or into terrain, handheld laser receivers are known which can highly accurately determine and indicate a position relative to a reference surface spanned by the rotation laser.
Handheld laser receivers known from the prior art for the determination of a position relative to the reference surface may in this case have a laser beam detector comprising a multiplicity of photosensitive elements, which is formed in order to generate an output signal when the laser beam strikes the laser beam detector. In detail, the laser beam detector is usually configured in this case so that an incidence position of the laser beam on the laser beam detector surface can additionally be derived, to which end the photosensitive elements—as considered in an upright operating position of the device—may be sequenced with one another in a vertically oriented sensor row, so that the laser beam detector thus extends at least over a one-dimensional region on the laser receiver. Furthermore, an evaluation unit for determining the position of the laser receiver relative to the reference height defined by the rotating laser beam with the aid of the output of the laser beam detector, as well as an indicator for the position determined (for instance a visual display), in particular formed in order to indicate whether the laser receiver coincides exactly with the reference surface, are usually integrated into the laser receiver device. The position may, for example, in this case be determined with the aid of a ratio of a plurality of output signals (for example as the midpoint of that subregion on the laser beam detector row which is illuminated by the laser beam).
Handheld laser receivers of this kind may, in particular, be used when the line imaged by the rotating laser beam is visible to the eye only with difficulty or not precisely enough. This is the case, for example, in the event of sizeable distances from the rotation laser (for example due to divergence of the laser beam [→ the imaged line becomes too wide] or a low light power (which is subject to certain limits for eye safety reasons) [→ the imaged line becomes too weakly visible] and/or high ambient brightness) or when using laser light in the nonvisible wavelength range.
In such cases, with the aid of such laser receivers it now becomes possible to find the laser beam and indicate or read the laser plane (or reference height) defined by a rotating laser beam, and to transmit the height information into the terrain or onto a wall (etc.). For example, a corresponding marking—indicated by the laser receiver—may be applied at the reference height.
To this end, on the user side, the laser receiver is for example moved up and down in a search pattern in the vertical direction and finally brought into the position in which the indicator displays coincidence with the reference surface. As the indicator, for example, a visual display may be provided which gives information (for example by luminous arrows or different-colored LEDs) about whether a defined zero point of the laser receiver (for example a surface midpoint of the detector surface):                lies exactly at the height of the reference surface,        lies above the reference surface or        lies below the reference surface.        
Examples of such laser receivers are disclosed in documents EP 2 199 739 A1 and U.S. Pat. No. 4,240,208.
In order to provide the user with simple transmission of the reference height determined and indicated by the laser receiver, a height mark may be provided on the housing of the laser receiver at the height of the defined zero point (for example a notch or a printed line laterally on the housing).
U.S. Pat. No. 7,394,527 discloses a system consisting of a laser emitter and a laser receiver, the intention being to determine a distance of the laser receiver from the laser emitter. To this end, it is proposed to emit two mutually parallel laser beams in a rotating fashion and to determine the distance as a function of the rotation speed and time offset of the laser pulses of the two laser beams, which are received directly after one another. In a similar way to this—when there are a plurality of detector strips offset parallel to one another on the receiver (with an accurately known parallel offset of the detector strips from one another)—as an alternative, a single laser beam may also be emitted in a rotating fashion, in which case the distance from the receiver to the laser emitter is determined as a function of the time offset of the laser pulses received successively by the respective detector strips.
U.S. Pat. No. 5,953,108 discloses a system consisting of a rotation laser and a laser receiver, the laser beam rotating with a first speed when no information is being transmitted to the laser receiver, and with a second speed different to the first rotation speed so as thereby to transmit predefined information concerning a status of the rotation laser (for example “low battery”).
For a range of known functions and applications of a system consisting of a rotation laser (in particular a dual-grade rotation laser) and a laser receiver, knowledge (sometimes approximate) about a laser receiver direction may additionally be necessary or at least helpful, that is to say knowledge about a direction in which the laser receiver lies in sight of the rotation laser (for example with respect to a coordinate system internal to the rotation laser).
Examples of such functions and applications may in this case be:
a) Grade catch (also known as plane or slope catch), for example when the receiver is lost from the locked-in state (see b)):
A search of the receiver is carried out by varying inclination of the reference plane spanned by the rotating laser beam and finding as a function of a hit signal emitted from the receiver to the rotator.
Contact Point of the Function with Laser Receiver Direction:
If an azimuthal direction to the receiver is known (for example from a previous locked-in state), then the search process can be accelerated with the aid of this laser receiver direction (less uncertainties existing for the conduct of the search process). If it is furthermore known whether the laser receiver has left the plane from the locked-in state upward or downward, then—in turn with the aid of the laser receiver direction—the search process can again be carried out more expediently.
b) Grade lock (also known as plane lock or slope lock, optionally with tracking), which can only be carried out when the reference plane has already (at least somewhat) impinged on the detector region of the receiver, i.e. in particular directly following a grade catch (that is to say after “finding” the receiver):
The reference plane is locked in at the zero point of the receiver (that is to say controlled inclination of the reference plane so that it intersects the zero point of the receiver) and this state is optionally continuously held (for example, even if the receiver moves, which is then known as tracking), it being ensured continuously by controlled readjustment of the reference plane inclination that the zero point of the receiver is touched by the reference plane, or cuts the reference plane).
Contact Point of the Function with Laser Receiver Direction:
In the event of dynamic movement of the receiver, the tracking (that is to say the following of the reference plane by controlled inclination thereof) can only be carried out sufficiently rapidly with direct knowledge of which deviation, measured on the receiver side, between the current reference plane incidence point and the zero point of the laser receiver, needs to be reacted to with which inclination adjustment. To this end, knowledge about the laser receiver direction can be very helpful for stabilizing this tracking process and making it more direct, and therefore permitting faster and more dynamic tracking.
c) Axis alignment/axis finding:
Fictitious x′ and y′ axes (that is to say fictitious axes which do not correspond with the orientation, dictated by setup and design, of the actual x inclination and y inclination axes of the core module), about which the laser plane is intended to be inclined as input by the user, may be defined on the user side. This user-side input of the desired orientation of the fictitious x′ and y′ axes may be carried out with the aid of a current direction to the receiver, which is to be determined (so that, for example, the fictitious x′ axis can be placed in this azimuthal direction to the receiver).
Furthermore, for the purpose of the axis alignment, an assistance functionality may also be implemented in such a way that signaling in relation to the quality of the orientation of the inclination axis is carried out, for instance by a display which assists the orientation of the laser core module, effected on the user side (for example a value indication or left/right/middle information). This is advantageous in particular when, owing to design, the inclination system does not permit provision of fictitious x′ and y′ axes, as for instance in rotation laser systems of low development levels with exclusive horizontalization function (self leveling), which do not have means for mechanical rotation of the inclination axis/axes.
Contact Point of this Function with Laser Receiver Direction:
For this function (at least when defining the fictitious x′ and y′ axes by means of a current direction to the receiver), knowledge about the laser receiver direction is necessary.
Special aspects and embodiments relating to these functions are described, for example, in the patent literature publications U.S. Pat. No. 6,055,046 A, U.S. Pat. No. 6,314,650 B1 and U.S. Pat. No. 6,693,706 B2.
Inter alia, the following methods are in this case known in the prior art (inter alia from the publications mentioned in the section immediately above) for the determination of a laser receiver direction in a system consisting of a rotation laser and a laser receiver:
1) Evaluation of a signal generated directly (in real time) after detection of a beam on the receiver side, this signal being transmitted from the receiver to the rotator (for example by radio), and derivation of an emission angle which the rotating laser beam was probably at the time of incidence.2) Defined inclination of the reference plane by a known inclination angle and reading, on the laser receiver side, of a thereby caused height offset of the beam strike on the detector of the laser receiver (with these steps being carried out for both inclination axes) and derivation of a direction to the receiver with the aid of the given relation of the respective inclination angle difference to the respective height offset on the receiver.3) Attachment to a beam parameter of the laser radiation of continuously angle-dependently varying information which can be read on the part of the receiver with the aid of the incident beam and furthermore makes it possible to determine the direction to the receiver.4) Iteratively halving windowing as a function of a hit or non-hit of the laser receiver in the respective current angle range window (for example emission of the beam only in the angle range of 0-180° if the receiver has displayed a hit: emission of the beam only in the angle range of 0-90° if the receiver has not displayed a hit for 0-180°: emission of the beam only in the angle range of 180°-270°, etc.).
The topic relating to determination of the laser receiver direction is in this case dealt with, inter alia, in the patent literature publication WO 2006/070009 A2.
In practice, however, the known methods for determining the direction have been found to be slow, not very stable, not very reliable and/or elaborate or difficult to practically implement.
Methods as described under point 1) above, which are based on the transmission from the laser receiver to the rotation laser of a signal which depends on the time of the strike on the laser receiver, and derivation of the direction information carried out directly therefrom (that is to say a real-time observation), are not very accurate (since the signal transmission time depends on the respective distance between the receiver and the rotator) and are not reliable.
The methods as described under points 2) and 4) above are relatively laborious to carry out and require various different steps and/or decisions, so that they therefore also lead to a relatively high error susceptibility. Furthermore, the method as described under point 2) functions only with a receiver held upright (oblique holding of the receiver leads to a vitiated result).
The method as described under point 3) is very elaborate to carry out, since it is necessary to attach continuously varying information to the laser radiation and, furthermore, on the receiver side special acquisition of the incident laser beam is also required (that is to say so that the attached information is not lost) and evaluation with respect to the information has to be carried out. In addition, during the evaluation of a signal generated upon detection of the laser radiation, a continuously angle-dependently varied beam parameter of the laser radiation can lead to inaccuracies in the final angle determination (for example when a global shift of the parameter in its value, which is varied angle-dependently, occurs (for instance owing to shock or ageing of the device)).